(1) Field of the Invention
The present invention relates to a surface-treating method of screw parts which method comprises coating beforehand a highly insulating organic resin based coating composition onto a thread of screw parts to be mounted onto an automobile body by welding.
(2) Description of the Background Art
In the field of an assembly of an automobile, for example, a fastening method by use of a combination of a nut with a welded bolt, or a bolt with a welded nut has been known in the art. That is, a normal fastening method comprises mounting the bolt or the nut by welding onto a body, chassis, suspension and the like, followed by coating and fastening with a nut or bolt respectively, and the normal fastening method has widely been used as a method having high workability, because welding of either the bolt or the nut makes the use of a detent unnecessary. After mounting the bolt or the nut by welding, however, spatters, i.e. molten metal droplets due to a welding operation carried out therearound may be scattered and adhered onto the thread of the screw parts, or a catianic electrodeposition coating film formed as an undercoating for the automobile body is thickly deposited on the thread, resulting in producing on fastening various troubles such as an abnormal increase in a screwed torque, development of an abnormal biting and the like because of resistance to fastening by the adhered spatter and the deposited electrodeposition coating film. For the purpose of preventing the above troubles, there is known in the art a method of masking the thread so that adhesion of the spatter onto, or penetration of the coating composition into the thread of the bolt or nut may be prevented. However, the above method has such drawbacks that in the actual production line, the application and removal of a mask require many manhours, resulting in reducing an assembly efficiency, and in that development of a more efficient method is demanded.
For the purpose of solving the above problems, there is also known in the art a method of coating beforehand a tetrafluoroethylene powder onto the thread of the screw parts. However, the method has such a drawback that a coated film shows poor properties in adhesion properties and wear resistance and shows difficulties in use because of easily falling off.